The present invention relates to the multiple modes of inhibition by a drug molecule of an enzymatic pathway postulated for a disease state. In particular the invention relates to new chemical compounds that inhibit the various enzymes in the arachidonic acid pathway implicated in inflammatory disease conditions.
Arachidonic acid is present in cell membranes as esters of phosphorylated glycerides, the so-called phospholipids. Under some defined biological stimulus, certain lipases can hydrolyze, either directly or in stepwise fashion, these phosphorylated arachidonic glycerides to release free arachidonic acid. Of the lipases that perform such function, most preeminent is the enzyme Phopholipase A2; This enzyme hydrolyzes the phospholipids and arachidonic acid is released in a single step.
Once arachidonic acid is released into the cell constituents, there are abundant opportunities for it to undergo transformation into several products. There are five major pathways that regulate the fate of arachidonic acid.
The first pathway is reincorporation of arachidonic acid back onto phospholipids mediated by a group of enzymes including arachidonoyl-CoA synthetase and arachidonoyl-CoA:lysophospholipid transferase;
The second pathway is the transformation of arachidonic acid into pro-inflammatory prostaglandins. Such a transformation is brought about by a group of enzymes called cycloxygenases which are further subdivided mainly into cyclooxygenase-1 and Cycloxygenase-2. While the former is understood to be a constitutive enzyme, the latter is inducible. This pathway generates pro-inflammatory prostaglandins. By inhibition of cyclooxygenase enzymes by suitable substrates, one can exert remedial action of inflammatory reactions. There are several types of anti-inflammatory compounds designed to inhibit cycloxygenases some of them being more selective for Cox-2 than for Cox-1. Foremost among them are the so-called NSAIDs (Non-steroidal anti-inflammatory drugs) have been recognized to be safe over extended periods of usage. Drug molecules like Ibuprofen and Naproxen are among the prominent members of this class of drugs. Several of them belong to the aryl propionic acid structural types.
The third pathway is mediated by lipoxygenases which transform arachidonic acid into hydroperoxy-eicosatetraenoic acids (HPETE). Of the three lipoxygenases (5-LO, 12-LO, 15-LO), the most prominent is 5-Lipoxygenase pathway leading to the formation of leukotrienes. Zileuton is an example of 5-lipoxygenase inhibitor, belonging to the N-hydroxyurea class of compounds. 5-Lipoxygenase operates in a much more complex way. For 5-lipoxygenase to be effective, it needs the cofactors Ca2+, ATP and another protein nicknamed 5-FLAP (5-Lipoxygenase activating protein), a membrane protein. 5-Lipoxygenase relocates to the membrane binding with the three cofactors mentioned and starts the transformation of arachidonic acid eventually to result in leukotrienes. Inhibitors of the 5-FLAP are effective anti-inflammatory compound switching off the 5-lipoxygenase chain. Among the 5-FLAP inhibitors may be mentioned, MK-866, (±)2-fluoro-a-methyl-1,1′-Biphenyl]-4-acetic acid. (Novel Inhibitors of Leukotriene, Edited by G. Folco, B, Samuelson, R. C. Murphy, Birkhläuser Verlag, Basel, 1999)
The fourth pathway is again a metabolic one, for example, happening in neural cells where cytochrome P450 converts arachidonic acid into an epoxide, Epoxyeicosatrienoic acid which further is converted to diols through the agency of epoxide hydrolases. Similar type of oxidative transformations also occur with other family members of
The fifth pathway is diffusion of arachidonic acid outside the cell.
Finally arachidonic acid can also be transformed into its ethanol amide, called Anadamide, identified as an endogenous ligand for cannabinoid receptors in brain cells.
The present invention is designed to inhibit the two major pathways of arachidonic acid, namely cyclooxygenase pathway and 5-Lipoxygenase pathway. Since these two pathways lead pro-inflammatory compounds, simultaneous inhibition of such pathways will lead to enhance efficacy of compounds. By designing suitable compounds which will inhibit both the enzymes, the objectives of reducing inflammatory responses are achieved.
The present invention aims towards synthesis of two molecules connected by a molecular tether in such a way that the tether can be removed by enzymes such as proteolytic enzymes or esterases present in the cell cycloplasm or outside of it. Even with the tether in tact on the molecule, the molecules are configured to inhibit the individual intended enzymes. Boswellic acids have been known to be 5-lipoxygenase inhibitors through a non-redox route. The invention relates to the synthesis of molecules wherein the molecular components are brought together by linkages that can cleave by endogenous enzymes such as proteases, esterase and other hydrolytic enzymatic interventions.
3-Acetyl-11-keto-β-boswellic acid (AKBBA) whose structure is shown below is a potent 5-lipoxygenase inhibitor. It contains a free carboxylic group. The invention described herein transforms the free carboxylic group into a halomethyl ester, in particular to the chloromethyl ester. Such an ester, whose structure is shown within this application, serves as anchor for attaching other potent drug molecules such as Ibuprofen and Naproxen. The choice is not limited to these two only. Other arylpropionic acids can be linked as effectively as these two compounds    3-Acetyl-11-keto-β-boswellic acid (AKBBA)
    3-O-Acetyl-9,11-dehydro-β-boswellic acid chloromethyl ester
In a variant of this general strategy, Boswellic acids in general can be transformed into other active esters also;
Another variation is represented by the active ester incorporating Boswellic acid diene structure represented herein.
Examples are given below to illustrate the synthesis of such compounds. The examples serve as model ones and do not limit the types of structures that can be prepared by adopting this general strategy
    3-Acetyl-β-boswellic acid-9,11-diene-chloromethyl ester
None of such active esters have been described in prior art. Such core structures possess the structural features for 5-Lipoxygenase inhibition. These active sters are condensed with cyclooxygenase inhibiting structures such as arylpropionic acid molecules.

In the structure ‘R’ represents the core structure of a cyclooxygenase inhibiting features